Yersinia pestis is classified by the Centers for Disease Control and Prevention (CDC) and the National Institutes of Health (NIH) as a Category A priority bacterial pathogen that will most likely be used in a bioterrorist attack. Y. pestis is the etiological agent of the plague (Black Death), a transmissible disease that has been responsible for millions of deaths throughout the course of history. Typically, humans contract plague after being bitten by a rodent flea that carries the plague bacterium or by handling an infected animal. Millions of people in Europe died from plague during the Middle Ages, when human homes and places of work were inhabited by flea-infested rats.
Although the natural occurrence of the disease is now relatively rare, the deliberate release of Y. pestis is a real threat. Dispersal will most likely be in the form of an aerosolized release over a populated area. The first signs of an attack will be outbreaks of pneumonic plague 1-4 days later. If left untreated, pneumonic plague is nearly always fatal. Y. pestis may be transmitted from person to person. Transmission may occur through infectious respiratory droplets from pneumonic cases of the plague, or even from inhalation from contaminated clothes.
The use of Y. pestis as a biological weapon is not without precedent. The Tartars, during the siege of the Genoese-controlled Black Sea port of Kaffa, hurled plague-infected corpses over the city walls into the huddled city. During World War II, the Japanese reportedly released plague-infected fleas over populated areas of China, which resulted in sporadic plague outbreaks. Recent technological advances have enabled Y. pestis to be directly aerosolized, which is considered to be the most likely way the agent would be dispersed. The World Health Organization (WHO) estimates that an aerosolized release of 50 kg over a populated city could cause 150,000 cases of pneumonic plague and 36,000 fatalities. Compounding this threat is the possibility of deliberately releasing engineered antibiotic resistant strains which were reportedly produced in the former Soviet Union. Consequently, the potential for public disruption and panic would be severe.